Histomorphometric Plasticity in Pectoral Muscles and Adipose Tissue of Redheaded Bunting in Response to Skeleton Photoperiod.

Synchronization to the environmental day-night cycle is the key to survival in many animals. For a photoinduced physiological response to occur, Critical Daylength (CD) is required for that species. Some studies have shown changes in pectoral muscles and adipose tissue of migratory birds under the influence of 'Continuous light Pulse' (CP) photoperiodic schedules, but data under the influence of 'Skeleton Photoperiod' (SKP) is still lacking. Two groups of Redheaded Buntings were exposed to SKPs along with one group on shortday control, to test their effects on histological changes via histomorphometric analysis of pectoral muscle fibers and adipose tissue. The SKP groups were designed such that one tended to be photosensitive, while the other tended to be photostimulatory. The tissues of interest were cryosectioned and stained with either haematoxylin-eosin or oil red-O (ORO) stain. Thereafter, microscopy and imaging were done. We measured pectoral muscle fiber thickness, neutral lipid droplet size in muscle fibers, adipocyte area, adipocyte number density/ 1000 µm2, and adipocyte area frequency. Significant differences were observed in the morphometry of muscle fibers and adipocyte parameters of these groups. Depending on the perceived daylengths, positive correlations were observed between the characteristics of these tissues. Previous studies on buntings demonstrated physiological changes under such light regimes, but here we present evidence at the histological levels through histomorphometric analysis. It is suggested that adipocyte fat storage precedes muscle hypertrophy. The results raise a possibility for involvement of endocrine factors between pectoral muscles and adipose tissue in this migratory bird.